It is standard to equip a machining lathe in an automated operation with a workpiece positioner that serves to take each machined workpiece out of the lathe when this machine is done with it and that loads a new unmachined workpiece in. As described in German patent document 3,329,619 such a positioner typically has a pair of grippers so oriented relative to a normally horizontal gripper axis that each gripper can be rotated about this axis into a position which itself is alignable by rotation of the entire gripper about another normally vertical positioner axis with the axis of the lathe.
Thus in a normal cycle one gripper of the positioner head takes an unmachined workpiece out of a magazine and, when the workpiece in the lathe is complete, the positioner head moves out and removes this finished workpiece with its other gripper which is for this purpose aligned with the lathe axis. Subsequently the positioner pivots the one gripper carrying the unmachined workpiece into the position formerly occupied by the other gripper which now holds the just-completed workpiece, and transfers this unmachined workpiece to the chuck. The positioner head then withdraws and deposits the finished workpiece in the appropriate takeoff place with the other gripper and picks up another unmachined workpiece with the one gripper.
How the gripper holds the workpiece is determined by the type of lathing operation, either face or chuck machining for short workpieces and center machining for long ones. In chuck machining the workpiece is held only by the jaws of the chuck at the headstock of the lathe and its face or portions remote from the headstock are machined. In center machining the workpiece is held by the chuck at the headstock and is also supported for rotation about the lathe axis and the longitudinal workpiece axis at a center at the tailstock so that its sides between the headstock and tailstock can be machined.
For chuck machining as described in German patent document 2,444,124 the workpiece is normally held between three or four jaws that move synchronously toward and away from a center and that engage around the end of the workpiece that is remote from the chuck once the workpiece is in the chuck. Thus as the workpiece is being transferred between the positioner head and the chuck, by being gripped by the jaws of the latter and released by those of the former, the positioner jaws are spaced around and centered on the lathe axis. For center machining, however, the workpiece is gripped as described in German patent document 3,446,702 from the side midway between its ends and the gripper's jaws move in a plane including the lathe axis, normally engaging the top and bottom side of the shaft workpiece. While in face or chuck machining the workpiece can be swung about a vertical gripper axis, for center machining the gripper axis must be parallel to the lathe axis, which itself is normally horizontal, so that the workpiece does not take up too much room as it is manipulated.
Clearly the two different types of lathing operations require different types of positioners. Accordingly when a machine is switched from the one style of operation to the other it is standard to replace the positioner heads and their actuators because of the differences between the two styles of gripping and moving.